A lithographic printing plate generally comprises a hydrophobic imaging area, which receives oily ink in a printing process, and a hydrophilic non-imaging area, which receives dampening water. A conventional lithographic process usually comprises steps of masking a presensitized (PS) plate, which comprises a hydrophilic support and a hydrophobic photosensitive resin layer, with a lith film, exposing the plate to light through the lith film, and then developing the plate to remove a non-imaging area with a developing solution.
Nowadays a computer electronically processes stores and outputs image information as digital data. A presensitized lithographic plate is preferably scanned directly with a highly directive active radiation such as a laser beam without use of a lith film to form an image according to a digital data. The term of Computer to Plate (CTP) means the lithographic process of forming a printing plate according to digital image data without use of a lith film.
The conventional lithographic process of forming a printing plate has a problem about CTP that a wavelength region of a laser beam does not match a spectral sensitivity of a photosensitive resin.
The conventional PS plate requires a step of dissolving and removing a non-imaging area (namely, developing step). The developed printing plate should be further subjected to post-treatments such as a washing treatment using water, a rinsing treatment using a solution of a surface-active agent, and a desensitizing treatment using a solution of gum arabic or a starch derivative. The additional wet treatments are disadvantageous to the conventional PS plate. Even if an early step (image-forming step) in a lithographic process is simplified according to a digital treatment, the late step (developing step) comprises such troublesome wet treatments that the process as a whole cannot be sufficiently simplified.
The printing industry as well as other industries is interested in protection of global environment. Wet treatments inevitably influence global environment. The wet treatments are preferably simplified, changed into dry treatments or omitted from a lithographic process to protect global environment.
For example, a presensitized lithographic printing plate comprises a hydrophilic layer comprising colloid such as silica provided on a lipophilic layer (described in International Patent Application Nos. 94/18005, 98/40212 and 99/19143). The plate was imagewise exposed to light to abrade the hydrophilic layer within the exposed area. A heat-sensitive presensitized lithographic plate comprises a water-soluble or hydrophilic overcoating layer provided on the hydrophilic layer to prevent abrasion dust from scattering (described in Japanese Patent Provisional Publication Nos. 2001-096936 and 2002-086946).
Further, a press development method comprises the steps of attaching an exposed presensitized printing plate to a cylinder of a printer, and rotating the cylinder while supplying dampening water and ink to the plate to remove a non-imaging area from the plate. Immediately after exposing the presensitized plate to light, the plate can be installed in a printer. A lithographic process can be completed while conducting a usual printing treatment.
A presensitized lithographic printing plate suitable for the press development method must have a photosensitive layer soluble in dampening water or a solvent of ink. The presensitized plate should easily be treated under room light to be subjected to a press development in a printer placed under room light.
A conventional PS plate cannot satisfy the above-described requirements.
Japanese Patent No. 2,938,397 (corresponding to European Patent No. 0770494, and U.S. Pat. Nos. 6,030,750 and 6,096,481) discloses a method for making a lithographic printing plate. The method uses an imaging element (presensitized plate) comprising on a hydrophilic surface of a lithographic based an image forming layer comprising hydrophobic thermoplastic polymer particles capable of coalescing under the influence of heat and dispersed in a hydrophilic binder and a compound capable of converting light to heat. The method comprising the steps of imagewise exposing to light the imaging element; and developing a thus obtained imagewise exposed imaging element by mounting it on a print cylinder of a printing press and supplying an aqueous dampening liquid or ink to the image forming layer while rotating the printer cylinder.
The imaging element can be treated under room light because the element has sensitivity within an infrared region.
Japanese Patent Publication Nos. 2001-277740, 2002-029162, 2002-046361 and 2002-137562 disclose presensitized lithographic printing plate in which microcapsules containing a polymerizable compound are dispersed in place of the thermoplastic polymer particles.
A Computer to Cylinder (CTC) method has been proposed to advance digitalization from the stage of the CTP method. The CTC method can prepare a lithographic plate on a cylinder of a press machine by merely exposing the plate to light corresponding to digital image data without conducting development or other processes after the exposing step. The printing can be conducted immediately after preparing the lithographic plate.
A presensitized lithographic plate for the CTC method preferably has a hydrophilic image-forming layer that can be changed hydrophobic within a heated area, or have a hydrophobic image-forming layer that can be changed hydrophilic within a heated area.
When heating a hydrophilic polymer having a carboxyl group that can be decarboxylated (e.g., a group corresponding to sulfonylacetic acid), the polymer is changed to hydrophobic by a decarboxylation reaction. A presensitized lithographic plate having a hydrophilic image-forming layer that can be changed to hydrophobic within a heated area can be formed by using the above-mentioned hydrophilic polymer (described in Japanese Patent Provisional Publication Nos. 2000-122272 and 2001-33949). The hydrophilic polymer is preferably cross-linked or used in combination with a cross-linked polymer to prepare a lithographic plate without development.
A presensitized lithographic plate comprises an image-forming layer containing thermally fusible polymer particles and a hydrophilic polymer (described in Japanese Patent Provisional Publication No. 2002-226597). The plate is imagewise heated to fuse the particles to form a hydrophobic area as well as a not heated hydrophilic area in the image-forming layer.
When heating a hydrophobic polymer having a sulfonimido, disulfone or a sulfonate ester group, the polymer is changed to a hydrophilic polymer having a sulfo group. A presensitized lithographic plate having a hydrophobic image-forming layer that can be changed to hydrophilic within a heated area can be formed by using the above-mentioned hydrophobic polymer (described in Japanese Patent Provisional Publication Nos. 10(1998)-282642, 10(1998)-282644, 10(1998)-282646, 10(1998)-282672 and 11(1999)-309953). The hydrophobic polymer is preferably cross-linked or used in combination with a cross-linked polymer to prepare a lithographic plate without development.
A conventional presensitized lithographic plate has a colored image-forming layer to confirm an image after processing the plate (after development) and before printing (mounting the plate on a press machine).
According to a CTP or CTC method, an image cannot be confirmed before printing (at the stage of imagewise exposure or heating), even if the image-forming layer is colored. In the CTP or CTC method, the entire image-forming layer is still colored before mounting the plate on a press machine, since the lithographic printing is developed on a press machine or processed without development. Therefore, a printing-out agent is usually added to a presensitized lithographic plate for the CTP or CTC method. The printing-out agent has a function of forming a visible image at the imagewise exposing or heating stage to confirm the formed image.
An example of the printing-out agent is a combination of a compound forming an acid, a base or a radical when the compound is heated with another compound having a color that can be changed when the compound is reacted with the acid, the base or the radical (described in Japanese Patent Provisional Publication No. 11(1999)-277927). Another example of the printing-out agent is a thermally decomposable dye that is decomposed at a temperature of not higher than 250° C. (described in European Patent Application No. 1300241).